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Its not as good or as accurate as weathering_one's M1A1....
Toby, I am flattered by your statement and also mildly embarrassed.
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I had a lot of help from the people here but I am the last one to say that mine might be more accurate. I have, however, looked at more pictures of M1's than I can count and just used a sort of averaging of what I saw to try to recreate a similar effect.
FIrst off, might I suggest that you use a different coloured backdrop. The yellow is particularly bad for distracting the eye from the main subject.
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I find that either white or a light to medium blue is best. Your results are definitely "in the ballpark" or zone of what you should want. The pictures that I based my weathering on show more accumulation on the angled front part of the skirt and more so at the back and around the drive sprocket. Actually, I need to put more in my actual sprocket to match the picture I have.
I won't comment too much more as I really can't tell the effect from some of the angles of the pictures. For a vehicle traveling at a modest speed over rougher terrain for a shorter period of time, I surmised that most accumulation would be on the lower surfaces of the skirts, bow plate and rear. As you move up the plate vertically the accumulation should be lighter (exception being around the sprocket). As I said, these are my interpretations based on pictures.
I use inexpensive artist acrylics for my weathering. They are applied with various things such as a piece of stencil sponge, a stencil brush with a flat top and stiffer bristles and what decorative painters call a stippler (also called a stipple brush or scrumbler). For directional splatters, like on the bottom of a skirt for a vehicle traveling higher speeds, I use either a thin liner brush or a artist fan brush. Like everything, they can take some practice to achieve the right effect. As for the paint, I use quite diluted washes and even then remove some paint onto a paper towel before applying it to the subject. I hope that you find some of this helpful.
Regards,
AJ